The present invention generally relates to a developer supply unit in an electrophotographic copying machine and, more particularly, to a warning device for providing an indication of the occurrence of an abnormality in the control of the concentration of a developer powder such as toner.
In general, the automatic control device for the toner concentration currently utilized in electrophotographic copying machines comprises a sensor for detecting the concentration of toner within a developing unit, and a toner supply unit for supplementing toner into the developing unit when the concentration of the toner detected by the sensor comes to be of a value lower than a predetermined value.
However, according to the prior art, in the event that an abnormality occurs in the sensor and/or the concentration control device to such an extent that one or both of them fail to operate properly, the toner replenishment will not take place even if the toner concentration within the developing unit is lowered. The continued performance of cycles of a copying operation while the toner concentration has been lowered will further lower the toner concentration within the developing unit with the consequence that not only will the quality of images reproduced on copy papers be deteriorated, but also if the developer employed is of two component type consisting of toner particles and carrier particles, some of the carrier particles will undesirably flow onto the photosensitive medium and into the interior of the copying machine. In particular, where the carrier particles flow onto the photosensitive medium and into the interior of the copying machine, not only does the developer lose its function, but also the copying machine itself will be adversely affected and will not operate properly.